Cannot change post's language

Can you check if this version works fine? https://github.com/wp-plugins/ceceppa-multilingua/archive/master.zip

Nope. It did not fix it.

Do you think “resetting” the whole plugin may work?

Can you enable the debug and check if there is some warning from my plugin?

Where can I do this?

Can you check if the last update fix your issue?
If not, open your wp-config.php file and change this line:

// Enable WP_DEBUG mode
define('WP_DEBUG', false);

to:

// Enable WP_DEBUG mode
define('WP_DEBUG', true);

and also add those:

// Enable Debug logging to the /wp-content/debug.log file
define('WP_DEBUG_LOG', true);

// Disable display of errors and warnings
define('WP_DEBUG_DISPLAY', false);
@ini_set('display_errors',0);

// Use dev versions of core JS and CSS files (only needed if you are modifying these core files)
define('SCRIPT_DEBUG', true);

after try to set the language to your posts and past me the content of the file wp-content/debug.log

Couldn’t generate the Log. Something must not be ok in the instalation.
Anyways, I uninstalled, and reinstalled and got these notifications:

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 90

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 263

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 263

Notice: Undefined index: in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 210

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 219

Notice: Undefined index: in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 210

Notice: Undefined index: in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 210

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 226

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 72

Notice: Undefined index: in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 210

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 219

Notice: Undefined index: in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 210

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 219

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 72

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 263

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 263

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 263

Notice: Undefined index: in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 210

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/ceceppaml.php on line 325

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 72

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 311

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 311

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 263

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/ceceppaml.php on line 328

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 72

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/ceceppaml.php on line 331

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/ceceppaml.php on line 334

Notice: Undefined index: in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 210

Notice: Trying to get property of non-object in /home/content/30/8540930/html/Athletessite/wp-content/plugins/ceceppa-multilingua/includes/api.php on line 219

Can you check if last version fix your issue?

Hi,
I have a similar problem. I’m not able to change the title and the text of posts in multilanguage. I see on the right, of the editing post page, the flags in wich edit the title, but it puts the same in all language, so the content of the post. Maybe have I to insert a string of code in php of template post?
The website I work on is https://www.querciarossa.net/wordpress
If you notice, the home page in italian has “qualcosa in più” its content comes from home-service.php, but in the other languages there isn’t this part. How can I solve?
Many Thanks Alessandro